Static frequency-changing systems



March 1, 1960 N. S. PRYWES STATIC FREQUENCY-CHANGING SYSTEMS Filed Dec. 28, 1955 2 Sheets-Sheet 1 March 1 1960 Filed Dec. 28, 1955 INVENTOR. I\I UAH SIRYN es United States Patent STATIC FREQUENCY-CHANGING SYSTEMS Noah S. Prywes, Pennsauken, NJ.

Application December 28, 1955, Serial No. '555,809

Z Claims. (Cl. 321-68) This invention relates to static frequency-changing systems, and particularly to systems using magnetic elements for obtaining frequencies that are multiples of a supply frequency.

In certain of the prior-art frequency-changing systems saturable magnetic elements are used. Harmonics of a supply frequency are obtained by periodically saturating the magnetic elements. A tuned circuit or lter circuit may be used for separating a desired harmonic from the many other harmonics produced in the output circuit. Such systems are relatively inelicient when compared with conventional transformers because (l) a relatively large proportion of the supply power is used in saturating the elements, and (2) much of the supply power appears as undesired harmonics in the output circuits. The efficiency of such systems also falls off rapidly'when harmonies of a high order are desired because the output power decreases while the losses increase. Thus, in such systems, for a given amount of output power, relatively large amounts of magnetic material are required tobe Still another object of the present invention is to provide a novel static frequency-changer which has two distinctl modes of operation without changing thev supplyin one mode, furnishing output power at a desired multiple of the supply frequency and, in the other mode having substantially no output, and which can be triggered from one to the other of the two modes by a suitable triggering excitation.

A further object of the present invention is to provide an improved static frequency-changing system having advantages from the standpoint of reduced size, weight and cost, increased efficiency, and increased output power.

The above and further objects of the present invention are carried out by arranging a plurality of magnetic circuits to provide a resultant magnetizationV characteristic having two substantially straight vertical sides paral-V lel to the flux axis and located one on either side of the current axis, and a substantially flat, horizontal portion parallel to the current axis and joining the two vertical sides. An output circuit, including a capacitance element, is linked to the magnetic circuits. A magnetic device so arranged has at least two different modesof operation. In one mode, a suitable supply current, applied to input windings linking the magnetic circuits, produces 4changes only along the flat horizontal portion of, the characteristic, and substantially no outputv is n- Y magnetic circuits.

Patented `Mar. l, 1960 Mice duced in the output circuit. In a second mode, produced by applying a suitable triggering excitation, the same supply current maintains relatively large amplitude oscillations between twopoints, one on each of the two vertical portions of the characteristic. Substantial output at a desired multiple of the supply frequency is obtained in the second mode. Another suitable triggering excitation can be used to change the operation back to the one mode. Odd multiples of thesupply frequency can be obtained by linking the supply and output windings in series-aiding andthe bias windingin series-opposition to the magnetic circuits; or fractional, even, and submultiples of the supply frequency can be obtainedk by linking the supply windings and bias windings in series-opposition and the output windings in series-aiding to the By series-aiding is meant that a current flow in the thus connected windings applies magnetizing forces of like polarity to both magnetic circuits. By series-opposition is meant that a current flow in the thus connected windings applies a magnetizing force ofone polarity to one magnetic circuit and applies a magnetizing force of the opposite polarity to the other magnetic circuit.

The novel features and advantages of the present invention will be described in detail in connection with the accompanying drawing wherein:

Fig. l is a schematic diagram of a static frequency changer, accordingl to, the invention, for obtaining odd integral multiples of a supply frequency;

Fig. 2 is a graph, somewhat idealized, of the flux versus current characteristic of a system according to the present invention;

Fig. 3 shows two idealized graphs of the magnetization characteristic of a system according to the invention, one graph being taken for positive values of supply current, and the other graphbeing taken for negative values of supply current;

Fig. 4 is a graph of waveforms, allon the same Vtime scale, of supply current Is, output current lo, and flux.

A Fig. l;

Fig. 5 is a schematic diagram of an embodiment of the invention showing the .connection of the supply windings for obtaining even or `submultiples of a supply frequency;

Fig. 6 is a schematic diagram of another embodiment of the invention similar to that of Fig. 1 but modified in` that'it has the supply source connected across the output windings; y

Fig. 7 is a schematic diagram of a magnetic element, including a` permanent magnet in the magnetic circuit thereof and which. may .be used in a frequency-changer system according to the invention, and Fig. 8 is a schematic diagram of a three-legged mag netic element which provides a pair of magnetic circuits and which may be used in a frequency-changing system according to the invention.- Y

Referring to Fig. l, a frequency-changing system 10 includes ltwo separate magnetic circuits, for example the two separate magnetic cores 12 and 12. The cores 12 and 12' may have substantially similar dimensions, though not necessarily so, and may be made from conventional saturable magnetic material. A preferredmagnetic material is characterized by having a relatively low coercive force and having a Vrelatively sharp saturation VOther magnetic materials, such as `deltarnax, ferrite material, andV conventional transformer materials also may be used. l Each of the cores12` and 12' is linked respectively by. a supply winding 16 and 16', a bias winding 18 and -.18', andan output lwinding 20k and 20. In addition to the last-mentioned three windings, one of the cores 12 and 12', for example the core 12', also may be linked by a start winding 36. The supply windings 16 and 16 are connected in series-aiding by connecting the terminal 16b of the winding 16 to the terminal 16a of the winding 16. The remaining two terminals 16a and 16b of the supply windings 16 and 16 are connected to a supply source 22. The supply source 22 ymay be any suitable constant-current source. The bias windings 18 and 18' are connected in series-opposition in a bias circuit respectively including in series a direct-current source, illustrated by the battery 23, a current-limiting element such as a resistor 24, a choke such as the inductance element 25, and a normally-closed, double-pole, singlethrow switch 26. The terminal 18h of the bias winding 18 is connected to one terminal, the positive terminal, of the battery 24; and the terminal lb of the bias winding 18 is connected to the other, the negative terminal of the battery 24. The other terminals 18a and 18'a are respectively connected to the two fixed terminals of the Switch 26. The output windings 20 and 20 are connected in series-aiding by connecting the terminal Zilb of the winding 20 to the terminal 20'b of the winding 20. A capacitor 28 and a load are connected across the other terminals 29a and 20a of the windings 20 and 20.

The multiple frequency is delivered by the frequencychanging system to the load 30. The series load connection is the one prepared for a low-impedance load; in case of a high-impedance load, the load is preferably connected in parallel with the capacitor 2S. A battery 32 and a normally open, double-pole, single-throw switch 34 may be connected in series across the output winding terminals 26a and Ztla. One terminal, the positive terminal, of the battery 32 may be connected to the terminal 20a of the winding 20'; and the other terminal, the negative terminal, or" the battery 32 may be connected in series with the switch 34 to the terminal 20a of the winding 2t). The terminals 36a and 36b of the start winding 36 may be connected in series with a normally open, single-throw, double-pole switch 39, to a start pulse source 38. The start pulse source 38 may be any source adapted to furnish current pulses to the start winding 36. A `normally open, Single-throw, double-pole switch 47 may be connected across the capacitor 28.

The three double-pole, single-throw switches 26, 34 Iand 39 are used to illustrate three different means for initiating oscillations in the output circuit, including the output windings 20, 20 and the capacitor 28 of the system. Once initiated, such oscillations are maintained by the energy supplied by the supply source 22. The' output circuit oscillations can be terminated, for example, by momentarily closing the fourth double-pole, singlethrow switch 47 connected across the capacitor 28.

A characteristic magnetizing curve for the system of Fig. 1 is shown in Fig. 2 by the curve 40. The curve 40 is somewhat idealized in that the small hysteresis effect is neglected. The curve 40 may be obtained in a conventional manner by connecting an alternating power supply in the output circuit and observing the total ux changes and the output current Io. The illustrated curve 40 is obtained with the bias source 24 connected in the circuit. If the bias source 23 were disconnected, as by opening the switch 26, the characteristic 40 would be compressed along the Io axis and would resemble the conventional Z-type characteristic. The characteristic 40 has two regions of substantial current saturation, shown as the regions a and b on either side of the output current axis Io. By current saturation is means that large changes of ilux in the cores 12 and 12' produce small changes of output Io in theoutput circuits. The regions c and d of the characteristic 4l) represent ux-saturated conditions when both magnetic circuits are substantially saturated with flux, oriented in either the clockwise or counterclockwise senses. vIn certain prior-art systems, material having the conventional Z-type characteristic is employed and operation is carried out in the non-linear region of the knees of the characteristic. In other prior-art systems, a bias is used; however, the supply ampere-turns are required to be in excess of the bias ampere-turns in order to reach a non-linear region of the characteristic. In the latter systems, a linear inductance and a capacitor are included in the output circuit and are tuned to the desired multiple frequency. In such prior-art systems the eiciency, i.e., output power supply power, is relatively low because of the large supply ampere-turns required. This large supply ampere-turns is required in order to reach the non-linear regions. In the present invention the supply ampere-turns (ISNS) are much smaller, say live t0 ten times, than those of the bias ampere-turns (IBNB) as only a parametric change in exclusively linear regions is required. At the same time the output ampere-turns (IGND) yare approximately equal to the bias ampere-turns. In addition, a linear inductance element for tuning or filtering is neither required nor desirable in the output circuit. Accordingly, the eliciency of the present invention is appreciably better than that of prior-art systems. Also the efficiency remains substantially constant even when operating at higher frequency multiplication ratios and is of the same order of efciency as a conventional transformer.

Referring again to Fig. 2, the output at the desired frequency shifts back and forth between the two current-saturated regions a and b, as described more fully hereinafter. The region f between the two current-saturated regions a and b represents an intermediate condition when the algebraic sum of the ilux in the two similar circuits is substantially at a zero value.

The point 54 in the current-saturated region a of the characteristic 4i) represents the case when one core is saturated and the other has substantially zero ilux. In such case the output circuit has an output current I0'. The point .76 in the current-saturated region b represents another value of ilux where the other core is saturated and the one has substantially zero flux. In such case the output circuit has an output current -I. r[The values of output current I0 and -Io are approximately equal to u In, 2

where IB, IS are respectively the bias and supply-current amplitudes, and Nb, Ns, and N0 are respectively the number of turns of the bias, the supply and the output windings linked to the magnetic circuits.

If conventional transformer material is employed, the bias lampere-turns is made sufliciently large to obtain the desired non-linearity between the zero flux and the current-saturated regions.

The operation of the system of Fig. 1 may best be explained in connection with the idealized graphs of the magnetizing characteristics of Fig. 3 and the waveforms of Fig. 4. The characteristics 57 and 58 of Fig. 3 represent two idealized graphs of the magnetizing characteristics for the system of Fig. 2. The solid graph 57 represents the magnetizing characteristic during the interv al when the supply current Is is at its maximum posi-- tive value -l-is and the dotted characteristic 58 repre- .sents the magnetizing characteristic when the supply current IS is at its maximum negative value ISL- ip For convenience of drawing and explanation, eachrof the regions of the characteristics 57 and 5S is shown by a straight line. Oscillograms of the supply current IS, the output current Io and the flux si), all on the same time scale, areV shown in Fig. 4. One cycle of the supply current Is is shown by the waveform 60. The supply current IS has a positive value of -l-s and a negative value of is, and has a frequency fs. Five cycles of output current Io are shown by the waveform 62 of Fig.

`4. The output current is square Wave and has a maximum positive value of -l-io and a maximum negative value of -io, and has a frequency of 5 fs. The wave` form 64 showslive cycles of ilux I1 in the circuit corresponding to the ve cycles of output ciurrent Io of the waveform 62.

`During operation, oscillations in the output circuit are started by applying a momentary triggering excitation to themagnetic circuits. Priorto the initiation of oscillations, the supply current Is produces changes only in the zero flux region of the characteristic, for example, theregion f of the characteristic 40 of Fig. 2. The zero ux condition is a stable condition and represents one mode of operation of the system. Thus, in this condition substantially no output is delivered to the load device 30 of Fig. 1. i

The triggering excitation can operate as a pedestal to whichthe supply is added, bringing the operation from the zero flux region to one of the current-saturated regions. For example, the triggering excitation can be applied during the positive phase of the supply current IS, thereby moving the operation point to the upper currentsaturated region of the solid characteristic 57. The triggering excitation can then be removed and the output current Io thereafter oscillates between the currentsaturated regions ofthe characteristics 57 and 58. The pedestal type of triggering can be applied in the system of Fig. l either by momentarily closing kand then opening the single-throw switch 34 in theoutput circuit, or by closing the start switch 39 and activating the start pulse source 38 to apply positive, constant-current start pulses, in the direction of the arrow, to the start winding 36.

The ytrigger excitation also can operate to compress the zero flux region along the I0 axis of the characteristie. In such case the supply current Is will then have sufficient amplitude to change the operating point from the zero flux region to a current-saturated region. Upon removal of the latter triggering excitation, the characteristic expands to its initial width and the output current I0 oscillates between the two current-saturated regions of the characteristics S7 and 58. The latter type triggering excitation may be applied in the system of Fig. 1, for example, by momentarily opening and then closing the switch 26 connected in the bias circuit.

In general, the magnitude of the triggering excitation is made sufficiently great either to charge the capacitor 28 in excess of a maximum amplitude Vc, as described hereinafter, or, in the case of the start pulses to supply a saturation flux @s as described hereinafter. i

Thesecond mode of operation of the system, with larg oscillations at the multiple frequency in the output circuit, is also stable, and output power at the desired multiple frequency is delivered across the output circuit of Fig. l. The supply source 22 supplies the power absorbed in the output circuit and the system losses, mainly resistive losses, at the lower supply frequency fs. Substantially no power is required from the bias source 23.

, The shaded areas 65 and 67 between the two characterstic curves 57and 58 of Fig. 3 represent, respectively, the energy supplied to the output circuit when the supply current changes from y-is to +is, and vice versa. The fluxes IY and d represent the magnetic flux in the circuitf at the time when the supply current changes. The output'current Io changes abruptly in step fashion when the supply current IE switches from +s to -s, and from The steps 68 of theoutput current Io waveform 62 indicate the abrupt increase in the positive phase of output current I0 when the supply current changes from --is to -l-s, and the step 69 represents the abrupt change .in the output current Io when the supply current l,l changes -from -l-i, tol-is. The amount of 6 output current change represented by the steps 68 and 69 is approximately equal to a value NB 21Ns Thus, each time the supply current I,s changes phase, a parametric change from one to the other of the characteristics 57 and 58 is produced and energy is added to the output circuit. It is these added increments of energy which enable stable oscillation and enable output energy to be delivered to the load device. l

The waveform 64 of the ux I indicates that positive uxes are produced when the supply current Is changes from negative to positive, and negative fluxes P are produced when the supply current changes from positive to negative. The flux 1 waveform 64 shows a dampening effect during each .half cycle of supply current, with the flux i being repeatedly restored to its initial amount each time the supply current changes phase.

The magnetic energy of the circuit is stored in the capacitor 28 of Fig. l when the magnetic flux I is changing. Thus, the output circuit operates in a manner somewhat analogous to a LCR tank circuit, with the stored energy being switched back and forth between the magnetic circuits and the capacitive element at the desired multiple frequency.

- In order to maintain the proper relations between the supply-current (Is) changes and the circuit flux (1P) changes, the output current Io is required to oscillate at an odd integral multiple of the supply frequency fs.

Y The following equations are approximate, but are sufficiently accurate for use in designing. a frequencychanging system yas described in Fig. 1:

where C is equal to the value of the capacitor 28 in farads; No is the number of turns of the output windings 20 and 20.

where Vc max is the maximum voltage, in volts, developed across'the capacitor 28, and

(4) MF4@ nismo where Io is the average value of the output current io.

' The circuit of Fig. 1 can be arranged to supply desired even integrals, fractional and sub-integrals of the supply frequency by changing the connection of the supply windings 16, 16 thereof from series-aiding toseries-opposition relation. Fig. 5 shows the series-opposition connection of the supply windings with the terminals 16a, 16a ,connected to each other, and the terminals 16h, 16b connected to the supply source 22. The remaining winding connections to the cores 12 and 12 may be the same as those of Fig. 1. In the case of the series-opposition connection offthe supply windings, the flux changes in the cores'12 and 12 are in phase with the changes of the supply current is. Accordingly, stable operation is produced when the output frequency nfs is an even integral, a fractional integral, or a sub-integral of the supply frequency fs.

' In the embodiment of Fig. 6, a supply source is connected across the output windings Eiland 20 of the cores 12 and 12'. One output terminal of the Supply source is connected to the one terminal 20a of the output winding 20, and the other output terminal of the supply source is connected to a source of reference potential, indicated in the drawing by the conventional ground symbol. The terminal Ztl'b of the output winding 20 is connected in series with the parallel circuit comprising a capacitor 75 and a load 76 to a terminal `77 connected to the common ground. The bias windings 18 and 18 are connected in series-opposition relation to a directcurrent bias source 70.

The supply source may be, for example, any suitable one arranged to furnish a sinusoidal waveform 81 at a frequency fs. The output waveform is then substantially square wave, as indicated by the waveform 83, at the desired multiple n of the supply frequency fs. A bypass iilter 74 is connected across the terminals 20a and ground to bypass the multiple frequency (nfs) currents of the output circuit from the supply source. In practice, the tilter 74 may consist of a cage in the rotor of the supply generator.

Oscillations may be initiated in the output circuit by momentarily interrupting the bias current IB, or by charging the capacitor 75, or by a separate start winding, as described for the system of Fig. l. The system can be switched to the non-oscillating, for example, by interrupting the supply current Is, or by momentarily short-circuiting the capacitor 75, as by momentarily closing the double-pole, single-throw switch 78 connected across the capacitor 75.

The cores 12 and 12 of the systems of Figs. l and 6 each may be replaced, if desired, by a composite core 80 of Fig. 7. The core Si? has a permanent magnet portion 82 for supplying the bias flux. By using a composite magnetic core, the bias source and the bias windings may be dispensed with. The permanent magnet portion 82 is bonded to the remaining portions of the core 80 so as to minimize the air-gaps. A composite core, such as the core St), is known in the art. The volume of the permanent magnet portion Si) is made suiciently large to produce a biasing ux equal to the bias magnetizing force NBIB of Figs. 1 and 6.

The two magnetic cores 12 and 12 of the systems of Figs. l and 6 may be combined into a single three-legged magnetic core 84, as shown in Fig. 7. The core 84 has two separate magnetic circuits each about one of the apertures 85 and 86. The respective supply and output windings may be linked to the outside legs of the core 84 and a single bias winding may be linked to the center leg. Also, if desired, a permanent magnet portion may be included in the center leg of the core 84.

There has been described herein an improved frequency-changing system which provides advantages by way of efficiency', size and cost. Odd, even, fractional, or submultiples of a supply frequency may be obtained. The output frequency consists essentially of the desired frequency, and additional ltering or tuning elements are not required. The circuit is stable in either of two modes. A supply either does or does not produce output frequency depending upon the mode selected. In one mode a substantial output is obtained and, in the other mode, substantially no output is obtained. The system can be triggered back and forth between the two modes by applying suitable triggering excitations. A relatively large output can be delivered to a load in an eicient manner.

What is claimed is:

l. A static frequency-changing system comprising a pair of magnetic circuits, means for biasing said circuits in opposite directions of magnetization, a supply circuit for applying alternating vmagnetizing forces simultaneously to said circuits, an output circuit including energystoringr means connected to said magnetic circuits for storing energy supplied to said circuits by said supply circuit, said biasing magnetization being substantially greater in magnitude than the magnetizations produced by said alternating magnetizing forces, and triggering means connected to said system for applying an electrical pulse to initiate electrical oscillations in said output circuit.

2. A frequency-changing system comprising a pair of magnetic circuits, a pair of supply windings respectively linked to said circuits for receiving an alternating input of one frequency, an output circuit including in series a pair of output windings respectively linked to said circuits and a capacitor, means for applying a bias magnetizing force simultaneously to said magnetic circuits, and triggering means connected to said system for applying an electrical signal to said circuits, said signal operating to induce an alternating output of a frequency different from said one frequency in said output circuit, and a load device connected in said output circuit.

3. A frequency-changing system comprising a pair of magnetic circuits, a supply, a bias, and an output winding for each magnetic circuit, means for applying an a1- ternating supply current of one frequency to said supply windings, means for applying a bias current to said bias windings, an output circuit including in series said output windings and a capacitor, and triggering means connected to said system for applying an electrical pulse to said magnetic circuits, said electrical pulse operating to initiate alternating currents in said output circuit at a frequency different from said supply frequency.

4. A static frequency-changing system comprising a pair of magnetic cores, a pair of supply windings respectively linked to said cores for receiving alternating supply currents of one frequency, a pair of bias windings respectively linked to said cores for receivinga bias current, the magnetizing forces applied to said cores by said bias current and bias windings being substantially greater in magnitude than the magnetizing forces applied to said cores by said alternating supply signal and said supply windings, an output circuit including a pair of output windings respectively linked to said cores and a capacitor connected in series with said pair of output windings, winding means coupled to Vat least one of said cores, and means for applying an electrical pulse to said winding means for inducing an alternating signal in said output circuit, the frequency of said output signal being different from the frequency of said supply signal.

5. A static frequency-changer for changing the frequency of alternating-supply currents to a desired odd integral of said supply frequency, comprising a pair of magnetic circuits each having supply and output windings, corresponding ones of said supply and output Windings being respectively connected in series-aiding, means for biasing said circuits to produce a magnetization characteristic having current-saturated regions respectively located on either side of the ux axis of said characteristic, an output circuit including said output windings and a capacitor, triggering means connected to said circuits for a rst electrical pulse to said circuits to initiate alternating currents at said desired frequency in said output circuit, whereby said output-circuit currents are maintained by energy derived from said supply currents and second triggering means connected to said circuits for applying another electrical pulse to said circuits to stop said output circuit oscillating currents.

6. A Static frequency-changer as described in claim 5, wherein said pair of magnetic circuits are included in a three-legged magnetic core.

7. A static frequency-changer as described in claim 5, wherein said pair of magnetic circuits consists of a pair of magnetic cores.

8. A static frequency-changer as described in 'claim 5, wherein said biasing means includes a separate bias winding for each of said magnetic circuits, and said bias windings being connected in series-opposition.

9. A static frequency-changer as described in claim 5, wherein said biasing means includes a permanent magnet in each of said pair of magnetic circuits.

.10.- A static frequency-changer for changing an alternating supply frequency to a desired one of an even multiple, a fractional multiple or a sub-multiple of said supply frequency comprising, a pair of magnetic circuits each having a supply and an output winding, said supply windings being connected in series-opposition relation and said output windings being connected in series-aiding 1l. A static frequencychanger as described in claim Y 10, including a load device connected to said capacitor.

12. A static frequency-changer as described in claim 10, wherein said pair of magnetic circuits consist of a pair of magnetic cores. f

13. A static frequency-changer as described in claim 10, wherein said pair of magnetic circuits are included in a three-legged magnetic core.

14. A static frequency-changer comprising a pair of magnetic circuits, a pair of supply windings respectively linked to said circuits in the same one sense for receiving supply currents of one frequency, an output circuit including a pair of output windings respectively linked to said circuits in Said one sense and a capacitor, a pair of bias windings respectively linked in the one sense and the sense opposite the one sense to said magnetic circuits, said pair of bias windings being arranged for receiving a unidirectional bias current, said supply currents and said supply windings and said bias current and said bias windings being so proportioned that the supply ampere-turns of magnetizing force applied to said circuits are appreciably less than the bias ampere-turns of magnetizing force, triggering means connected to said circuits for selectively initiating oscillations at a desired frequency in said output circuit whereby said oscillations are maintained by energy derived from said supply currents at said one frequency, and second triggering means connected to said circuits for selectively stopping said oscillations in said output circuit.

15. A frequency-changing system comprising a pair of magnetic circuits, a supply,a bias, and an output winding for each magnetic circuit, means for applying an alternating supply current of one frequency to said supply windings, means for applying a bias current to said bias windings, an output circuit including in series said output windings and a capacitor, triggering means connected to said system for applying a rst electrical pulse to at least one of said magnetic circuits, said first electrical pulse operating to initiate in said output circuit alternating currents of a frequency different from said one frequency, said alternating currents being maintained in said output circuit by energy derived from said supply currents, and second triggering means connected to said system for applying a second electrical pulse to said at least one of said magnetic circuits, said second electrical pulse operating to stop any alternating currents of said desired frequency in said output circuit. l

16. A frequency-changing system as describe-d in claim 15, wherein corresponding ones of said supply and output windings are connected in series-aiding relatiomand said bias windings are connected in series-opposition relation.

17, A frequency-changing system as described in claim 15, wherein corresponding ones of said supply and bias windings are connected in series-opposition relation, and said output windings are connected in series-aiding relation.

18. A frequency-changing system comprising a pair of magnetic circuits, means for supplying alternating magnetizing forces of one frequency simultaneously Vto said circuits, means for applying a unidirectional biasing magnetizing force simultaneously to said circuits, said biasing magnetizing force being appreciably greater in magnitude than said alternating magnetizing forces, means including winding means linked to said circuits and a capacitor for deriving alternating currents at a frequency different from said one frequency from said circuits, triggering means connected to said system, and means for applying an electrical pulse to said triggering means for initiating said alternating currents.

19. A frequency-changing system as described in claim 18, wherein said means for supplying said alternating currents includes said winding means, and a filter connected across said winding means for bypassing said alternating currents of said different frequency.

20. A frequency-changing system as described in claim 18, wherein said means for supplying said alternating magnetizing forces includes a pair of supply windings respectively linked to said circuits.

References Cited in the le of this patent UNITED STATES PATENTS 1,586,885 Elmen June 1, 1926 1,645,303 Slepian Oct. 11, 1927 1,862,211 Dowling June 7, 1932 2,150,386 Manley Mar. 14, 1939 2,418,642 Huge Apr. 8, 1947 2,721,304 Silver et al. Oct. 18, 1955 2,725,520 Woodworth Nov. 29, 1955 FOREIGN PATENTS 714,146 Great Britain Aug. 25, 1954 

